The present invention relates to a liquid-vapor contact column having a plurality of trays supported within an outer tubular shell to contact liquid-vapor phases with one another. More particularly, the present invention relates such a column in which the trays are staggered to prevent a counter-flow pattern for the vapor phase as it ascends between trays of the column.
Liquid-vapor contact columns are commonly used in distillation applications. Other applications include heat exchange, absorption of a gas in liquid etc. In liquid-vapor contact columns, trays are supported within the column in order to contact gaseous and liquid phases, for example, phases of a mixture to be distilled. In the cryogenic distillation of air, as a vapor phase ascends within a column, it becomes evermore enriched in the lighter components of the air, such as nitrogen. As the liquid phase descends within the column, it becomes evermore concentrated in the heavier components of the air, for instance, oxygen. There are different types of tray designs that are useful in contacting liquid and vapor phases together. Commonly, in cryogenic air separation, sieve trays are used. Also used are bubble cap and expanded metal trays.
The liquid phase descends within the column, from tray to tray, through downcomers. Liquid flows from the downcomers onto an inlet portion of a tray and then across an active portion of the tray where liquid-vapor contact occurs. The liquid is then discharged from an outlet portion of the tray. The outlet portion discharges liquid to the next downcomer that opens up to the next lower tray. Vapor ascends through the column through the active portions of the trays which are provided with openings for the vapor to flow up through the tray and contact the liquid phase.
The flow pattern of the vapor phase, as viewed from tray to tray, interacts with flow pattern of the liquid phase. As the vapor ascends through the liquid, it does not ascend at right angles to the tray, but rather, is deflected at an angle to the tray, in the direction of the liquid flow. When trays are spaced so efficiently close together, instead of continuing the ascent to the next tray, at this angle, a counterflow pattern or path develops in which the vapor phase reverses direction. The effect of the curvature of the stream lines is a pressure gradient in the horizontal direction, with the highest pressure at the liquid outlet portion of the tray. This produces a higher hydraulic gradient of liquid which is necessary to overcome this pressure gradient and as result, froth heights are higher near the liquid inlet. Moreover, the backwards flowing vapor may carry entrained liquid backwards which increases the liquid load on the tray towards the inlet portion of the tray. This can lead to pre-mature flooding of the tray. The resultant higher hydraulic gradient may also lead to weeping near the liquid inlet which deteriorates the mass transfer efficiency because the weeping liquid by-passes two trays.
As will be discussed, the present invention provides a distillation column in which the trays are arranged to reduce the vapor cross-flow by straightening the counterflow path of the vapor phase between trays, thus allowing smaller tray spacings and more compact distillation column designs.